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Spring 2008 Subsidence and Geologic Field Observations <br />Box Canyon, Apache Rocks, and South of Divide Mining Areas <br />• <br />alluvium) of Sylvester Gulch above mined Longwall Panels 14 through 16, where the overburden <br />depth to the B-seam ranges from 900 to 1,600 feet. <br />The length of time between the formation and healing of cracks (crack duration) is a function of <br />their location with respect to the mine geometry, the type of material in which they form, crack <br />width and depth, and annual precipitation. Healing occurs as a result of erosion, mass wasting, <br />deposition, infilling, and revegetation. Cracks that form above moving longwall mining faces <br />tend to close again when the longwall face moves out of the area of mining influence. Crack <br />duration, in areas of permanent tension, such as above solid coal boundaries or rigid chain pillars, <br />is summarized (from earlier annual observation reports) as follows: <br />1. Cracks in colluvium commonly heal and revegetate in about one to three years. <br />2. Cracks in soft, friable bedrock, such as the soft sandstone above mined Longwall Panel <br />13 (Apache Rocks mining area), are no longer visible in roughly three to six years. <br />• <br />• <br />3. Cracks in hard, durable bedrock, such as the sandstone outcrop at Apache Rocks, will <br />likely be visible for many decades. <br />No mining effects on rockfalls or landslides were observed in the Apache Rocks mining area. <br />However, fresh-looking cracks and scarps were observed during field visits since 2006 <br />suggesting that local, sporadic falls and slides continue in the rockfall/landslide area near the <br />head scarp of the first east drainage of Sylvester Gulch in the Box Canyon mining area. <br />Rockfalls and landslides in this area and above mined Longwall Panels 19 to 22 are categorized <br />as having a very high rockfall potential. Rockfall and landslide activity were notably accelerated <br />during the mining of Longwall Panels 19 to 22. <br />A new, active rockfall/landslide area was observed on the west side of West Flatiron during the <br />spring 2007 field visit. The area is described in this report as Location 5 (Map 1) and is located <br />in the headwater region of Box Canyon and in one of the most narrow parts of West Flatiron <br />ridge at a site where probable gravity-induced cracks were first observed. However, this area is <br />now actively cracking and slabbing above the mined part of Longwall Panel 21. Cumulative <br />831-032.790 <br />August 2008 <br />Wright Water Engineers, Inc. <br />Page 2